An established part of the manufacture of tires is testing the tires for uniformity. After the tires have been manufactured, they are routed to a uniformity measuring machine. One example of a tire uniformity testing machine is described in U.S. Pat. No. 6,016,695, “Tire Uniformity Testing System”, which is incorporated herein by reference. A typical uniformity machine will automatically convey the tire into the machine, chuck it between two half-rims, inflate it, load it against a drum to simulate the weight of a vehicle, proceed with steps to test it, then mark the tire and sort the tire into one of several output conveyors. Loadwheel assemblies found in currently available uniformity testing machines are described in U.S. Pat. No. 5,979,231 “Loadwheel Assembly for Tire Testing Systems Having Conical Support Plates”, and U.S. Pat. No. 4,704,900 “Apparatus and Method for Imposing a Desired Average Radial Force on a Tire”, both of which are incorporated herein by reference. Some uniformity machines perform additional steps, such as grinding the tire to improve its characteristics and measuring geometric parameters of the tire.
The testing steps on a typical uniformity machine consist of rotating the loaded, inflated tire, measuring the forces exerted by the tire on the drum as a function of the rotational position of the tire, performing calculations on those measurements to obtain measurements of the characteristics of the tire, reversing the rotation of the tire (a description of a spindle assembly that is used to mount and rotate the tire during testing is found in U.S. Pat. No. 5,992,227 “Automatic Adjustable Width Chuck Apparatus for Tire Testing Systems” which is incorporated herein by reference), repeating the measurement process, comparing the measurements of the characteristics in one or both directions to preset limits to obtain grades for the tire's uniformity and determining the output conveyor for further processing, placing these grading and sorting results into memory for later use in marking and sorting, and stopping the rotation at the correct orientation so that a mark can be placed at a required angular location on the tire, usually corresponding to the high point of the first harmonic of radial force variation.
One common measure of the quality of a uniformity machine is its repeatability, how well the measurements of the tire characteristics repeat when tires are tested multiple times. Lack of repeatability can be caused by many sources on a machine, such as poor control of the test conditions of load and inflation, excessive runout in the test rims or loading drum, electrical noise, mechanical vibrations, etc. Because repeatability is such an important measure of a machine's quality, much attention has been focused into obtaining good repeatability over the years.
It has long been known that certain types of tire construction can lead to poor repeatability when tires of these types are used in the measurement of the repeatability of a uniformity machine. For instance, the use of nylon as a reinforcement material in the tire often leads to poor repeatability. Unfortunately, in recent years, more tires are being constructed in designs which cause poor repeatability, due to the increasing emphasis on the reliability of tires through the use of components such as belt edge overlays, and the increasing importance of high speed rated tires, which very frequently use a nylon cap ply. The increasing prevalence of these difficult tires presents a problem for the manufacturer of tire uniformity measuring machines. The machines continue to be of the same high quality as in years past (or even of improved quality), but the measurement of their repeatability with test tires indicates that the machine's quality is not up to standard.
Furthermore, the measurement of these difficult tires presents a problem for tire manufacturers. If there is a large uncertainty in the measurement of their characteristic (at least when measured with a commercially viable cycle time), the tire manufacturer cannot reliably grade the tire into the correct category, resulting in possibly dissatisfied customers or lost revenue due to downgrading a tire unnecessarily.